TST: Move risk tests to their own module.

Will be adding the Excel spreadsheet answer key, a dedicatetd
directory for risk should help make clear why and where the spreadsheet
is used.
TST: Read benchmark returns directly from answer key spreadsheet.

The risk tests originally were based on a spread sheet, with the
results of returns etc copy and pasted into the `test_risk` module.

Include the spreadsheet and read the values directly using a Python
Excel spreadsheet library.
TST: Download risk answer key from S3.

So that the answer key does not onerous on the SCM repo size, add a
utility to download the answer key automatically.

Prevent re-download on every test suite run if the local answer key
matches the latest version.
Merge reading of risk unit test expected results from a spreadsheet.

To help tentpole the risk results with an alternative implementation.

Also, the spreadsheet provided is what the original answers were based
from, reading from said spreadsheet should help prevent drift.